However, I was mistaken. CannonGT sent me an email saying that the source code is actually still missing, as I reported originally. Unfortunately, legal issues aside, no one knows exactly where the code is:

CannonGT: We successfully negotiated permission to obtain it from Vivendi in 2006-2007, but Vivendi could not locate the files. The code may have been either lost or stolen, but we have certainly not acquired it. While I continue to pursue free download options, I have no active project to locate source code.

That's a bummer. But considering Red Baron 3d still has some sort of a community going strong after ten years really says a lot. Here's hoping to a worthy successor!